1. Field of the Invention
The present invention relates to an acceleration sensor for detecting acceleration, which is used for toys, automobiles, aircrafts, portable terminals and the like, and particularly to an acceleration sensor that can be produced using a semiconductor technology.
2. Description of the Related Art
Acceleration sensors utilizing a change in physical quantity such as a piezo resistance effect and a change in electrostatic capacity have been developed and commercialized. These acceleration sensors can be widely used in various fields, but recently, such small-sized acceleration sensors as can detect the acceleration in multi-axial directions at one time with high sensitivity are demanded.
Since silicon single crystal becomes an ideal elastic body due to the extreme paucity of lattice defect and since a semiconductor process technology can be applied for it without large modification, much attention is paid to a piezo resistance effect type semiconductor acceleration sensor in which a thin elastic support portion is provided at a silicon single crystal substrate, and the stress applied to the thin elastic support portion is converted into an electric signal by a strain gauge, for example, a piezo resistance effect element, to be an output.
As a three-dimensional acceleration sensor, an acceleration sensor has been used, which comprises elastic support arms each of a beam structure formed by a thin portion of a silicon single crystal substrate connecting a mass portion constituted by a thick portion of a silicon single crystal substrate in a center and a frame in its periphery. A plurality of strain gauges are formed in each axial direction on the elastic support arms. In order to sense a small acceleration with an enhanced sensitivity, the elastic support arms are made long and/or thin, or the mass portion that works as a pendulum is made heavy. The acceleration sensor that can detect a small acceleration has led to an excessive amplitude of the mass portion, when subjected to a large impact, and resulted to break the elastic support arms. To avoid the break of the elastic support arms even if a massive impact is applied, regulation plates have been installed above and below the acceleration sensor element to restrict amplitude of the mass portion within a certain range.
An acceleration sensor having regulation plates is described in Japanese Laid-Open Patents HEI 4-274005, HEI 5-41148 and HEI 8-233851.
Japanese Laid-Open Patents HEI 4-274005 and HEI 8-233851 also disclose a method in which, to control a gap at a predetermined value between the regulation plates and the mass portion of the acceleration sensor element, small balls having a diameter of substantially the same distance as a gap are mixed with adhesive, and the adhesive with small balls mixed is used to bond regulation plates to the acceleration sensor element. The gap can be maintained at a predetermined value because the gap between regulation plates and the acceleration sensor element can be dictated by a diameter of small balls. The use of adhesive containing small balls thus enables the control of a gap between regulation plates and the acceleration sensor element.
Acceleration sensors are manufactured by a process comprising: forming a number of acceleration sensor elements on a silicon wafer of about 6 inches in diameter by a photo lithography technology, cutting the wafer into acceleration sensor elements one by one, fixing each of the acceleration sensor elements in a protection case and connecting electrical terminals with conductors, mounting a regulation plate on the acceleration sensor element and fixing a lid of the protection case onto the protection case with adhesive.
The process for manufacturing the acceleration sensor elements from the silicon wafer uses a sputtering apparatus for forming terminals and lead wires on the elements, an ion implantation apparatus for forming piezoresistors, a dry etching apparatus for dry etching the silicon wafer and the like, besides coating and developing photo resistive films and rinsing them. Particularly in the dry etching step, the silicon wafer is fixed onto a dummy substrate with resin adhesive to cool the silicon wafer. Elastic support arms are liable to fracture during removing the resin adhesive after the dry etching step, since they are as thin as 5 to 10 μm, while they are of 500 to 700 μm long and 80 to 120 μm thick. The adhesive cannot be removed by applying a mechanical force but by using a solvent. By the reason, even a small amount of residual adhesive tends to remain on the elements. Also, there may be asperities or protrusions caused on the elements because dust is scattered in the sputtering step of lead wires. Most of the asperities or protrusions (hereafter referred to as “contaminants”) caused in the sputtering are less than some μm in height and of soft material so that they do not affect the bending of the elastic support arms nor reduce output voltages and sensitivity of the acceleration sensor, although a side of them is longer than 10 μm. Contaminants caused by sputtering are harder in material than the resin adhesive and as high as 0.1 to 5 μm in height, although they are as large as some μm to 10 μm in a side length of their area. But, the sputtered contaminants do not affect the bending of the elastic support arms. It is thought that the contaminants do not affect the measurement results of acceleration, since they are small in volume and in weight. It was proved that there are no contaminants larger than 5 μm high, when a lot of contaminants were measured in height.
The contaminants cannot be neglected with respect to a gap g between a regulation plate mounted to face an acceleration sensor element and a mass portion on the acceleration sensor element, since the gap is as small as about 15 μm. In view of the height of the contaminants, most of the acceleration sensor elements in which the contaminants adhere a mass portion of the element cannot be used. Because of that, those having contaminants on a surface facing a regulation plate had to be disposed of as a defective product. The contaminants lowered production yields, resulting in raising prices of the acceleration sensors.
In a usual acceleration sensor, a gap between non-wired areas of an upper surface of a mass portion and an upper regulation plate has a length of more by a thickness of lead wires on the acceleration sensor element than that between wired areas of the upper surface of the mass portion and the upper regulation plate. However, since the wire thickness on the sensor element is about 1 μm at most and the contaminants adhering the mass portion are as large as about 5 μm in height, there is an extremely high possibility of the contaminants contacting the upper regulation plate first, when the contaminants adhere any area on the mass portion and when the mass portion is displaced by excessive acceleration, and the output is apt to be saturated even if the applied acceleration is within a measurable range.